This invention relates generally to a system for the fusion of a plasma by energy beam illumination of a target pellet of plasma-forming material. More specifically, the invention relates to laser fusion reactors. Thermonuclear fusion reactors are a potential and prominent proposed solution to the long-term energy problem. One area of interest in fusion reactor research is the investigation of the potential for laser fusion. In this technique, fusion would be induced in a spherical pellet of deuterium and/or tritium containing material by illuminating it with an intense, coherent and, preferably, spherically symmetric pulse of light from a laser. The general theoretical process of inducing fusion by means of a laser is well-known and described in the prior art. Briefly stated, the energy of a sufficiently intense pulse of laser light focused onto the surface of the target pellet is absorbed by the surface of the pellet, causing the surface to vaporize and expand away from the center of the pellet at high velocity. The reaction force from this expansion compresses the remainder of the pellet to high density, and the combined effect of the illumination and compression heats the resultant plasma to a high temperature. If the combination of time of duration, temperature and density of the compressed plasma is sufficiently large, a so induced fusion reaction of the deuterium and/or tritium will generate more energy than was required to compress and heat the pellet, thereby accomplishing a net generation of energy.
Since the confinement of the plasma for a short time during the fusion burn is due to the inertia of the compressed pellet material, the above-described process is called Inertial Confinement Fusion (ICF).
The fusion reaction releases copious amounts of X-ray, ion, and neutron radiation. The radiation can be used to produce useful power by the heating of a coolant fluid and can also induce useful nuclear transformations in blanket and fuel assemblies located in the reactor. The neutrons can generate additional energy by inducing fission reactions in nearby fuel assemblies.
An ICF reactor need not be designed to be capable of net energy generation in order to be a useful device due to the above-mentioned nuclear transformations, especially transformations of nuclear species from fertile to fissile species.
A power reactor will utilize fuel pellets in rapid succession, with in excess of 10 pellets per second being a not unlikely rate of use. Despite this high use rate, the radiation flux is not continuous, but rather arrives at the surface of the reactor container (called the "first wall") and the fuel and blanket assembly surfaces in intense, brief bursts after each pellet implosion. The radiation flux can be destructive of the impacted surfaces.
The implosion of the pellet by laser light has been thought to require that the pellet be illuminated uniformly over its surface such that the pellet compression is also uniform. In old art, the pellet is simultaneously illuminated by several laser beams from beam sources mounted at intervals around a spherical reactor vessel, the pellet being located at the center. The beam sources are usually optical systems comprising mirrors and prisms which split a laser beam from a single laser into a plurality of beams which are then directed to the pellet from different directions. The pellet becomes a point source of plasma which emits radiation uniformly in all directions.
All known approaches to ICF devices have specified a single pellet implosion site. This has been required in part by the use of multiple beams of laser light from various directions to achieve uniform pellet surface illumination. Simultaneous beam arrival at a single site is most conveniently arranged when the pellet is located equidistant from all beam sources: that is, at the center of a spherical vessel.
Recent developments in the art indicate that less uniform illumination, even two-sided illumination, will suffice.
The fusion burn of a pellet at an implosion site at the center of the reactor vessel exposes the first wall, fuel assemblies and blanket assemblies to a point source flux of various types of radiation. This flux uniformly irradiates the first wall, fuel and blanket assemblies only if these members are shaped to present a spherical surface, centered at the implosion site, to the radiation flux. A uniform exposure of the first wall is desirable to most widely distribute the heat deposited therein, thereby reducing peak wall temperatures and reducing consequent wall erosion and other damage mechanisms. The uniform exposure of the blanket assemblies is desirable for these reasons as well, and also optimizes the production of nuclear isotopes. A uniform neutron flux in the fuel assemblies reduces peak temperatures and heat fluxes there, enhancing the usable life of the assemblies.
Despite the advantages of uniform radiation exposure, it is inconvenient to design the first wall, and fuel and blanket assemblies to be spherical or quasispherical. Engineering considerations suggest that cylindrical geometry for these members is preferred.
Consequently, it is an object of this invention to provide an ICF system which achieves an approximately uniform cumulative radiation exposure of the first wall, and fuel and blanket assembly members while avoiding the need to shape these as spheres.